This invention relates to a system for controlling an internal combustion engine, and more particularly to a control system in which the signals representing the amount of air supplied to an internal combustion engine, the temperature of the engine, the rotating speed of the engine, the load of the engine, and the composition of exhaust gases from the engine are processed by a microprocessor to obtain various control signals so that these control signals can be used for the control of the controlled variables of the engine, especially, for the control of the amount of fuel supplied to the engine.
A known disclosure, for example, U.S. Pat. No. 3,969,614 discloses a method for controlling an internal combustion engine in which a digital computer is used to control the controlled variables of the engine, including the amount of fuel supplied to the engine, the ignition timing and the amount of exhaust gases recirculated through the engine, on the basis of the results of detection on the amount of intake air supplied to the engine, the temperature of the engine, the rotating speed of the engine, the load of the engine, and the composition of exhaust gases from the engine.
In the steady running condition of a motor vehicle driven by an internal combustion engine, the amount of intake air supplied to the engine is the most important factor for controlling the amount of fuel supplied to the combustion chamber of the engine. The flow rate of intake air is detected by an air flow meter disposed upstream of the throttle valve, and an output signal representing the detected flow rate of intake air is delivered from the air flow meter.
In a fuel supply system in which fuel is supplied to an internal combustion engine in synchronous relation with the engine crankshaft position, a method is generally employed according to which the open period of the fuel valve is controlled to control the amount of fuel supplied to the engine. In this case, the open period of the fuel valve is controlled to lie approximately within the range of 2.5 ms to 9 ms. When a digital signal is used as this control signal for controlling the open period of the fuel valve, a binary-coded decimal signal of 12 bits (=4.times.3) will be enough to ensure the accuracy of control within 1%. Suppose that the minimum open period 2.5 ms of the fuel valve corresponds to a binary-coded decimal number 100. Then, the maximum open period 9 ms of the fuel valve will be less than a binary-coded decimal number 400, and the number of bits of the binary-coded decimal signal will be as many as 11 bits (=3+4+4). Suppose further that the digital signal is a binary-coded signal, and 256 (=2.sup.8) corresponds to the maximum open period 9 ms of the fuel valve. Then, the minimum open period 2.5 ms of the fuel valve will correspond to about 50, and a digital signal of 8 bits will be enough to ensure the accuracy of control within .+-.1% (=.+-.0.5/50). Therefore, a digital control signal having a limited number of bits as above described can be sufficiently used for the desired control of the amount of fuel supplied to the engine.
However, due to the fact that the output signal of the air flow meter has a level which is generally approximately proportional to the detected flow rate of intake air, the output signal of the air flow meter has a low level when the flow rate of intake air is small. Thus, when the output signal of such a low level is converted into a digital signal of a limited number of bits to be used for digital processing, a change in the flow rate of intake air in this region cannot be represented with high accuracy. In other words, the resolution for the flow rate of intake air is degraded in the small flow rate region when such a flow rate is represented by the digital signal of the limited number of bits. This fact will be discussed in more detail. Generally, the metering range of the air flow meter metering the flow rate of intake air is from about 0.1 m.sup.3 /min to about 5 m.sup.3 /min which is about 50 times the value of 0.1 m.sup.3 /min. Suppose that the flow rate of intake air is represented by a binary-coded signal of 10 bits, and the maximum flow rate of intake air 5 m.sup.3 /min is represented by 2.sup.10 =1024, then, the minimum flow rate of intake air which is about 0.1 m.sup.3 /min is represented by 1024/507/8207/82.sup.4 =16. Therefore, a high resolution of about 1/1000.times.100=0.1% is obtained in a region where the flow rate of intake air is large or close to its maximum, and a change in the flow rate of intake air can be indicated with high accuracy in such a region. However, the resolution is only about 1/20.times.100=5% in a region where the flow rate of intake air is small or close to its minimum, and a change in the flow rate of intake air within 5% cannot be indicated in such a region.
Therefore, in an engine control system in which the output signal of an air flow meter is converted into a digital signal of a limited number of bits and is then subjected to digital processing by a microprocessor to obtain a control signal for controlling the amount of fuel supplied to the combustion chamber of the engine, the control signal contains an insufficient amount of information when the flow rate of intake air is small, and the accuracy of control of the amount of fuel supplied to the combustion chamber of the engine is reduced in the region where the flow rate of intake air is small or close to its minimum. The control of the air-fuel ratio to maintain it at a proper value in the region of the small flow rate of intake air, that is, during driving a vehicle at low speeds is especially important from the viewpoint of obviating environmental pollution by the toxic components of engine exhaust gases, and such a reduction in the accuracy of control of the amount of fuel supplied to the combustion chamber of the engine must be avoided as much as possible. It is necessary to increase the number of bits of the digital signal representing the flow rate of intake air detected by the air flow meter in order to prevent the undesirable reduction in the accuracy of control in the region of the small flow rate of intake air. To this end, the microprocessor must have a parallel processing capacity with an increased number of bits, or the arithmetic processing time in the microprocessor must be extended when the parallel processing capacity of the microprocessor is not increased. The former is disadvantageous from the economical standpoint, and the latter is also disadvantageous from the standpoint of the control response, hence, the accuracy of control.
The prior art proposals have failed to refer to the problem of the forementioned reduction in the accuracy of control of the internal combustion engine in the region of the small flow rate of intake air, especially, the problem of the undesirable reduction in the accuracy of control of the amount of fuel supplied to the combustion chamber of the engine in such a region, and have also failed to provide any concrete solution for this problem.